This is still a bug in Steam that should be fixed: Steam is changing the global cursor theme of the root X window. After starting Steam (and only then), this weird inverse cursor is also appearing in some other places, namely when opening a context-menu in a GTK-based application (like Firefox or Thunderbird) - I am using KDE as desktop environment.Steam should obviously not mess with my global cursors...

Using Jimmac cursor theme on arch I had to replace /usr/share/icons/Jimmac/cursors/arrow symbolic link from right_ptr to left_ptr. Would be better to not change steam cursor to arrow as not everyone use it.